Finding All Duplicates in a List in Java- BAEL-6075 (#13291)

* Deep & Shallow Copy

* Revert "Deep & Shallow Copy"

This reverts commit 62f75b002a21feb90c0b6be02072356b64284283.

* duplicates in a list in java

* Unit test changes

* unit test changes

* Tests change after review

* Code formatted

* code formatted

* new module core-java-lang-6

* entry in parent pom and test setup method name change

* module renamed

* parent pom entry added

* pom.xml changes reverted

core-java-modules/core-java-collections-list-5/src/main/java/com/baeldung/listduplicate/ListDuplicate.java

@@ -0,0 +1,69 @@
+package com.baeldung.listduplicate;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+
+public class ListDuplicate {
+    public List<Integer> listDuplicateUsingSet(List<Integer> list) {
+        List<Integer> duplicates = new ArrayList<>();
+        Set<Integer> set = new HashSet<>();
+        for (Integer i : list) {
+            if (set.contains(i)) {
+                duplicates.add(i);
+            } else {
+                set.add(i);
+            }
+        }
+        return duplicates;
+    }
+
+    public List<Integer> listDuplicateUsingMap(List<Integer> list) {
+        List<Integer> duplicates = new ArrayList<>();
+        Map<Integer, Integer> frequencyMap = new HashMap<>();
+        for (Integer number : list) {
+            frequencyMap.put(number, frequencyMap.getOrDefault(number, 0) + 1);
+        }
+        for (int number : frequencyMap.keySet()) {
+            if (frequencyMap.get(number) != 1) {
+                duplicates.add(number);
+            }
+        }
+        return duplicates;
+    }
+
+    public List<Integer> listDuplicateUsingFilterAndSetAdd(List<Integer> list) {
+        Set<Integer> elements = new HashSet<Integer>();
+        return list.stream()
+          .filter(n -> !elements.add(n))
+          .collect(Collectors.toList());
+    }
+
+    public List<Integer> listDuplicateUsingCollectionsFrequency(List<Integer> list) {
+        List<Integer> duplicates = new ArrayList<>();
+        Set<Integer> set = list.stream()
+          .filter(i -> Collections.frequency(list, i) > 1)
+          .collect(Collectors.toSet());
+        duplicates.addAll(set);
+        return duplicates;
+    }
+
+    public List<Integer> listDuplicateUsingMapAndCollectorsGroupingBy(List<Integer> list) {
+        List<Integer> duplicates = new ArrayList<>();
+        Set<Integer> set = list.stream()
+          .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
+          .entrySet()
+          .stream()
+          .filter(m -> m.getValue() > 1)
+          .map(Map.Entry::getKey)
+          .collect(Collectors.toSet());
+        duplicates.addAll(set);
+        return duplicates;
+    }
+}

core-java-modules/core-java-collections-list-5/src/test/java/com/baeldung/listduplicate/ListDuplicateUnitTest.java

@@ -0,0 +1,68 @@
+package com.baeldung.listduplicate;
+
+import java.util.Arrays;
+import java.util.List;
+
+import org.junit.Assert;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+public class ListDuplicateUnitTest {
+    private static ListDuplicate listDuplicate;
+
+    @BeforeClass
+    public static void setup() {
+        listDuplicate = new ListDuplicate();
+    }
+
+    @Test
+    public void givenList_whenUsingSet_thenReturnDuplicateElements() {
+        List<Integer> list = Arrays.asList(1, 2, 3, 3, 4, 4, 5);
+        List<Integer> duplicates = listDuplicate.listDuplicateUsingSet(list);
+        Assert.assertEquals(duplicates.size(), 2);
+        Assert.assertEquals(duplicates.contains(3), true);
+        Assert.assertEquals(duplicates.contains(4), true);
+        Assert.assertEquals(duplicates.contains(1), false);
+    }
+
+    @Test
+    public void givenList_whenUsingFrequencyMap_thenReturnDuplicateElements() {
+        List<Integer> list = Arrays.asList(1, 2, 3, 3, 4, 4, 5);
+        List<Integer> duplicates = listDuplicate.listDuplicateUsingMap(list);
+        Assert.assertEquals(duplicates.size(), 2);
+        Assert.assertEquals(duplicates.contains(3), true);
+        Assert.assertEquals(duplicates.contains(4), true);
+        Assert.assertEquals(duplicates.contains(1), false);
+    }
+
+    @Test
+    public void givenList_whenUsingFilterAndSetAdd_thenReturnDuplicateElements() {
+        List<Integer> list = Arrays.asList(1, 2, 3, 3, 4, 4, 5);
+        List<Integer> duplicates = listDuplicate.listDuplicateUsingFilterAndSetAdd(list);
+        Assert.assertEquals(duplicates.size(), 2);
+        Assert.assertEquals(duplicates.contains(3), true);
+        Assert.assertEquals(duplicates.contains(4), true);
+        Assert.assertEquals(duplicates.contains(1), false);
+    }
+
+    @Test
+    public void givenList_whenUsingCollectionsFrequency_thenReturnDuplicateElements() {
+        List<Integer> list = Arrays.asList(1, 2, 3, 3, 4, 4, 5);
+        List<Integer> duplicates = listDuplicate.listDuplicateUsingCollectionsFrequency(list);
+        Assert.assertEquals(duplicates.size(), 2);
+        Assert.assertEquals(duplicates.contains(3), true);
+        Assert.assertEquals(duplicates.contains(4), true);
+        Assert.assertEquals(duplicates.contains(1), false);
+    }
+
+    @Test
+    public void givenList_whenUsingMapAndCollectorsGroupingBy_thenReturnDuplicateElements() {
+        List<Integer> list = Arrays.asList(1, 2, 3, 3, 4, 4, 5);
+        List<Integer> duplicates = listDuplicate.listDuplicateUsingCollectionsFrequency(list);
+        Assert.assertEquals(duplicates.size(), 2);
+        Assert.assertEquals(duplicates.contains(3), true);
+        Assert.assertEquals(duplicates.contains(4), true);
+        Assert.assertEquals(duplicates.contains(1), false);
+    }
+
+}